1. Field of the Invention
The present invention concerns the chromatic conversion of an image obtained by electromagnetic radiation. More particularly, but not exclusively, it addresses the field of radiology for the conversion of an x-ray image into a visible light image.
2. Discossion of the Background
FIG. 1 is a longitudinal cross-sectional view of a prior art device for the chromatic conversion of an x-ray image, showing the main constituent elements. The device T, also known as an x-ray intensifier tube XRIT, comprises a gas-tight envelope 1 having a surface of revolution 1, inside which is created a vacuum. The vacuum envelope 1 has an input faceplate 2 that is transparent to ionising radiation. Behind the faceplate 2 is located a photoelectric input screen 3 intended to receive the ionising radiation. In response to this ionising radiation, the photoelectric screen 3 emits electrons along electron trajectories 7. At the opposite end to the input faceplate 2 is located an output window 4 whose inner face contains an output phosphor screen 5 intended to receive the electrons and produce visible light in response therefrom by cathodoluminescence. The output window 4 is transparent to the radiation from the phosphor, thus allowing the derived image to be viewed.
The envelope 1 also contains focusing means 6, joined thereto by supports 64, for focusing the electrons emitted by the input screen 3 toward output screen 5. The focusing means 6 comprise a final electrode 60--also known as an anode--which can be brought to the same potential as the output screen 5 when the device is in operation, as well as a pre-terminal electrode 61 located in the region of the final electrode 60 and further removed from the output screen 5 than the latter. The focusing means 6 also comprise a set of electrodes 60 located between the pre-terminal electrode 61 and the input window 3.
FIG. 2, which also illustrates the prior art, is a partial longitudinal cross-section showing schematically first arrangement for fixing the output screen 5 with respect to the output window 4. The output window is formed of a transparent, or semi-transparent, plate 51 (transparency on the order of 0.8), generally made of glass, covered with a phosphor layer 50 that converts the energy of the incoming electrons. In general, this phosphor layer 50 is formed of a concentration of very small size grains. The plate 51 is normally tinted and its typical dimensions are on the order of 45 mm for the diameter and 0.6 mm for the thickness. The output screen 5 is separated from the output window 4 by a shell 53 such that a vacuum cavity 52 with parallel faces is formed between the plate 51 and the output window 4. However, this vacuum cavity 52 has a detrimental effect on the output image contrast. For this reason, it was eliminated in the output screen fixing arrangement shown in FIG. 3, which also illustrates the prior art. In this figure, the elements corresponding to--or having corresponding functions to--those of FIG. 2 are given references increased by 100 with respect to the latter. Only the differences between the figures shall discussed hereafter.
In the fixing arrangement of FIG. 3, the output screen 105 serves as an output window. The plate 151 bearing the phosphor layer 150 is relatively thick (on the order of 2.5 mm) in order to withstand the external pressure. This plate 151 is fitted with a mounting element 154 soldered by a string of solder 155 to a similar fixing element 112 forming part of the envelope 101. A first manufacturing step for this device would consist in depositing the phosphor layer 150 on the glass plate 151 and then soldering the glass plate 151 to the envelope 101.
However, this fixing arrangement has a number of drawbacks. Indeed, although the size of the plate 151 bearing the phosphor layer 150 is reduced, it is still too large and increases manufacturing costs, especially since this plate 151 must be sufficiently thick to withstand the external pressure. Furthermore, the output screen is brought to a very high voltage (around 30 kilovolts), and the mounting elements 154, 155 and 112 must be isolated from the outside environment by means of a substantial and complete insulating resin potting (not shown in FIG. 3). The constraints regarding the mechanical mount, the very-high voltage insulation, optical image relaying and manufacturing costs of the phosphor layer 150 are not all compatible with each other, and thus the optimum compromise for the whole system is not optimized for each aspect, with detrimental effects on costs.
Moreover, it is difficult to deposit an anti-reflection coating 174 on the external face of the output window, owing to its fragility. The procedure thus involves gluing an additional plate 173 onto the external face of the output screen 105 using a glue 172 having the same refractive index as both the plate 151 and the additional plate 173 so as obtain a uniform refractive index between the phosphor layer 150 and the anti-reflection coating 174. The anti-reflection coating 174, being fragile, is deposited only at the last stage of the product's manufacturing process.
There results a considerable increase in the number of successive operations in the device's manufacturing process, which again increases costs. Moreover, should the anti-reflection coating 174 become scratched, repairs are impossible without damaging the potting, which risks destroying the envelope 101.
To overcome these disadvantages, frequent use is made of the fixing arrangement shown in FIG. 4, which is also prior art. In the latter figure, elements that are similar to--or have similar functions to--those of FIG. 3 have references increased by 100 with respect to the latter. Only the differences between the two figures shall be described.
In this fixing arrangement, the phosphor layer 250 is deposited on a thin, light plate 251 having relatively reduced dimensions. This plate 251 is then held in optical contact with the internal face 240 of the output window 204 by means of a glue 208 having the same refractive index as both the plate 251 and the output window 204. Image contrast is consequently improved in comparison with the fixing arrangement of FIG. 2. A narrow passage 230 is provided in the envelope to accommodate a conductor 231 for supplying the very-high voltage to the screen, thus obviating the need for a complete potting.
On the other hand, the output screen 205 must be glued directly onto the internal face 240 of the output window 204. This creates manufacturing problems since the envelope has relatively large dimensions (height on the order of 200 to 400 mm) while the output window is located at the bottom of a shrunken portion of the envelope 201 whose average height and diameter are on the order of 50 mm and 80 mm respectively.
Moreover, should it be desired to deposit an anti-reflection coating 274 on the outside of the envelope 201, it is preferable to do so on an additional plate 273 glued onto the output window 204 by means of the glue 272 in order to avoid having to handle an assembled envelope of large dimensions.
The vast majority of optical assemblies used for viewing the optical image delivered by x-ray image intensifier tubes are set for an image behind a glass plate approximately 3.5 mm thick. However, new optical assemblies that are optimized for a plate thickness of at least 8 to 10 mm are being developed in view of improving image contrast. Their utilization means having to thicken either the plate 251 bearing the phosphor layer 250, or the output window 204, or the additional plate 273 bearing the anti-reflection coating. Owing to constraints regarding the production of the phosphor layer 250, it is undesirable to increase the thickness of its support plate 251. The same applies for the output window 204 when dealing with a generally large-size envelope 201. One solution would be to thicken the additional plate 273 supporting the anti-reflection coating 274. However, this has the drawback of making the additional plate 273 heavy, making it more prone to ungluing in the event of thermal or mechanical shocks. Moreover, with the fixing arrangement described in FIG. 3, the output screen 105 tends to be heavy and the mounting elements 154 must be designed in consequence while respecting the very-high voltage insulation requirements. This further increases manufacturing complexity and hence costs.